To save on the cost of detectors in a CT scanner, a large flat module detector has been introduced in the CT scanner. This large flat module detector has a plurality of flat modules, each module having a plurality of packs, each pack having a plurality of conventional detector cells. Compared to the traditional 3rd generation curved detector architecture, this large flat module detector is much larger, and is also different in many other ways. For example, in a large flat module detector, there exist air gaps between adjacent modules and between adjacent packs, while the traditional 3rd generation curved detector architecture contains no such air gaps.
Due to the actual manufacture and installation techniques of detectors, coherence between the actual sizes of these air gaps and the designed size thereof can hardly be guaranteed. Typically, coherence between the actual sizes of air gaps between adjacent packs and the designed size thereof is relatively easy to achieve during manufacturing. Moreover, because packs in a same module can be installed parallel to each other, the sizes of air gaps between adjacent packs will not be subjected to large errors during the installation. However, since the modules in the detector are typically installed obliquely at a certain angle relative to each other, stochastic errors will be introduced in the sizes of air gaps between adjacent modules, such that the sizes of air gaps between adjacent modules may be different in the same detector, and among different systems.
Furthermore, for a large flat module detector, obliquely incident X-ray photons may affect the effective response positions of the detector cells at the module edges, such that the effective response positions thereof are not at the centers of the detector cells, thereby affecting the sizes of air gaps between adjacent modules.
Due to at least these factors, the sizes of air gaps between adjacent modules in the large flat module detector become immeasurable. However, accurate air gap sizes, particularly the sizes of air gaps between adjacent modules, are very important for image reconstruction with a high image quality. Any mismatch between data acquisition and image reconstruction resulting from incoherence between the actual sizes of air gaps between adjacent modules and the designed size thereof may lead to severe ring artifacts in the reconstructed images.
For example, for a large flat module detector with a structure of 5 (the number of modules in the detector)×4 (the number of packs in each module)×34 (the number of detector cells in each pack), it is required that the size of air gaps between adjacent packs is 0.05 mm, and the size of air gaps between adjacent modules is 0.15 mm. In general, during the manufacture and installation of the detector, the errors introduced for the size of air gaps between adjacent packs are less than ±0.02 mm, while the errors introduced for the size of air gaps between adjacent modules are less than ±0.15 mm. When the errors introduced for the size of air gaps between adjacent modules are greater than ±0.04 mm, ring artifacts will occur in the reconstructed images, which is undesirable.
On the other hand, after the detector is secured to the gantry, its position cannot be adjusted. However, considering the precision of the mounting, the position of the detector may not correspond exactly to the desired position, which will affect the ISO channel. In other words, the actual ISO channel may not be the desired ISO channel. If an inaccurate ISO channel is adopted in the image reconstruction process, the Modulation Transfer Function (MTF) will be degraded. More seriously, double shadow artifacts may appear in the reconstructed images.
Considering that the abovementioned geometric parameters (e.g., the sizes of air gaps between adjacent modules, and the ISO channel) are immeasurable but critical to fulfill a high-quality reconstructed image, there is needed a method and apparatus for geometric calibration of a CT scanner.